The IUE MEGA Campaign: Wind Structure and Variability of HD 50896 (WN5)

We present preliminary results from an extensive study of UV line and continuum variability in the WN5 star HD 50896 = EZ CMa, monitored over 16 consecutive days in 1995 January, as part of the IUE "MEGA" campaign. Although variations are seen in all the major emission lines, the clearest occur in the unsaturated, subordinate N IV lambda 1718 line, and in the large number of Fe V/VI lines in the 1260--1450 A region. These and the continuum variations are clearly locked into the 3.766 day period, which has been known for this star since 1980. The observed variations suggest a global wind structure pattern that remains quite stable in the frame of the star, over the duration of the observations. This pattern is marginally compatible with that expected for the ionization cavity around an accreting neutron star companion in a 3.766 day W-R + NS binary. However, it can best be explained by some kind of corotating interaction regions emanating from hot (magnetically?) active regions near the surface of the stellar core. Such a model, although somewhat ad hoc, better accounts for the strong epoch-dependent variations seen in EZ CMa, as well as the fine structures seen in the global pattern.